A fluororesin sheet containing a filler is prepared by mixing a fluororesin with a filler and processing the mixture into a sheet form. As a result, the fluororesin sheet containing the filler not only has chemical resistance, heat resistance, non-adhesion properties and low friction properties, which are inherent in the fluororesin, but also has functions and properties, which are inherent in the filler. Moreover, the fluororesin sheet containing the filler is improved in wear properties and creep characteristics, which are the defects of a fluororesin. Accordingly, the fluororesin sheet containing the filler is variously used for sealing materials or the like.
Some of sealing materials are prepared by mixing a fluororesin with a metal, graphite or an inorganic filler so as to be improved in thermal conductivity, wear properties and creep characteristics. They are used as knitting yarns for sheet-like gaskets or gland packings.
U.S. Pat. No. 3,315,020 (Patent Document 1) discloses, in the specification, a technique such that a mixture of PTFE, a filler and a processing aid is extrusion molded and rolled by passing the molded mixture through a pair of pressure rolls once or several times, to prepare a long sheet. However, when the a sheet-like gasket is made from a fluororesin sheet containing a filler utilizing this technique, a gasket satisfying the properties as a sealing material cannot be prepared because the hermetical properties are low although the stress relaxation properties tend to be excellent if the filler content is high.
JP-A-2004-323717 (Patent Document 2) discloses a fluororesin sheet containing a filler prepared in such a way that a composition, which contains a fluororesin, a swelling mineral, a filler and a processing aid, is extrusion molded and thereafter stretched to be made into a sheet by passing the composition through a pressure roll. This fluororesin sheet containing the filler has excellent stress relaxation properties and tensile strength even if the fluororesin content is low and the filler content is high in the sheet.
It is found that the fluororesin sheet containing a filler disclosed in Patent Document 2 still has room for improvement in sealing properties because the amount of the fluororesin, which is used as a binder for filling the spaces between hard particulate inorganic fillers, is insufficient.
The present invention has been accomplished with the view of solving the problems as described above. It is an object of the invention to provide a fluororesin sheet containing a filler having both of high stress relaxation properties and high hermetic properties (sealing properties) even if having a low fluororesin content and a high filler content, and it is another object of the invention to provide a production process of the sheet.